Photovoltaic solar cells for directly converting radiant energy from the sun into electrical energy are well known. The manufacture of photovoltaic solar cells involves provision of semiconductor substrates in the form of sheets or wafers having a shallow p-n junction adjacent one surface thereof (commonly called the "front surface"). Such substrates may include an insulating anti-reflection ("AR") coating on their front surfaces, and are sometimes referred to as "solar cell wafers". The anti-reflection coating is transparent to solar radiation. In the case of silicon solar cells, the AR coating is often made of silicon nitride or an oxide of silicon or titanium.
A typical solar cell wafer may take the form of a rectangular EFG-grown polycrystalline silicon substrate of p-type conductivity having a thickness in the range of 0.010 to 0.018 inches and a p-n junction located about 0.3-0.5 microns from its front surface, and also having a silicon nitride coating about 800 Angstroms thick covering its front surface. Equivalent solar cell wafers also are well known, e.g. circular or square single crystal silicon substrates and rectangular cast polycrystalline silicon substrates. Amorphous silicon solar cells and other thin film solar cells are also known equivalents contemplated by this invention.
The solar cell wafers are converted to finished solar cells by providing them with electrical contacts (sometimes referred to as "electrodes") on both the front and rear sides of the semiconductor substrate, so as to permit recovery of an electrical current from the cells when they are exposed to solar radiation. These contacts are typically made of aluminum, silver, nickel or other metal or metal alloy. A common preferred arrangement is to provide silicon solar cells with rear contacts made of aluminum and front contacts made of silver.
The contact on the front surface of the cell is generally in the form of a grid, comprising an array of narrow fingers and at least one elongate bus (also hereinafter called a "bus bar") that intersects the fingers. The width and number of the fingers and busses are selected so that the area of the front surface exposed to solar radiation is maximized. Further, to improve the conversion efficiency of the cell, an AR coating as described overlies and is bonded to those areas of the front surface of the cell that are not covered by the front contact.
The rear contact may cover the entire rear surface of the solar cell wafer, but more commonly it is formed so as to terminate close to but short of the edges of the blank. Aluminum is preferred for the rear contact for cost and other reasons. However, the exposed surface of an aluminum contact tends to oxidize in air, making it difficult to solder a wire lead to the contact. Therefore, to facilitate soldering, it has been found useful additionally to provide apertures in the aluminum coating, with silver soldering pads being formed in those apertures so as to slightly overlap the adjacent aluminum layer.
The silver soldering pads form ohmic bonds with the underlying substrate and also low resistance electrical connections with the aluminum contact, and are used as sites for making soldered connections to the rear contact. The silver soldering pads are considered to be an integral part of the rear contact. Such a contact arrangement is disclosed in PCT International Publication No. WO 92/02952, based on U.S. patent application Ser. No. 07/561,101, filed Sep. 1, 1990 by Frank Bottari et. al. for "Method Of Applying Metallized Contacts To A Solar Cell". An alternative but similar back contact arrangement wherein the aluminum coating has apertures filled with silver soldering pads involves having the aluminum overlap the edges of the silver soldering pads.
The grid-shaped contact and the AR coating on the front surface may be formed in various ways, as exemplified by U.S. Pat. Nos. 4,451,969, 4,609,565, 4,751,191, 5,010,040, 5,074,920, British Patent No. 2,215,129, and PCT International Application WO 89/12321, published 14 Dec. 1989.
Regardless of how the front grid contact and the AR coating are formed, at least a portion of each bus of the front contact is not covered with the AR coating, so as to permit making a soldered connection to that contact.
Photovoltaic solar cells (e.g., silicon solar cells) are typically small in size, e.g., 2-6 inches on a side in the case of cells made from rectangular EFG-grown substrates, with the result that their power output also is small. Hence, for convenience of construction and assembly, industry practice is to combine a plurality of cells so as to form a physically integrated module with a correspondingly greater power output. Several solar modules may be connected together to form a larger array with a correspondingly greater power output. The cells in a module are electrically connected in parallel and/or in series, and two or more modules in an array may be connected in series or in parallel, depending on the voltage and current output that is desired from the combined modules.
A usual practice is to form a module from two or more "strings" of silicon solar cells, with each string consisting of a straight row of cells connected in series, and the several strings being arranged physically in parallel with one another. The several strings are electrically connected to one another in parallel or in series, according to voltage and current requirements. A common practice is to use solder coated copper wire, preferably in the form of a flat ribbon, to interconnect a plurality of cells in a string, with each ribbon being soldered to the front or back contact of a particular cell, e.g., by means of a suitable solder paste as described in U.S Pat. No. 5,074,920.
For various reasons including convenience of manufacture and assembly, cost control, and protection of the individual cells and their interconnections, it has been common practice to provide modules in the form of laminated structures. These laminated modules consist of front and back protective sheets, with at least the front sheet being made of clear glass or a suitable plastic material that is transparent to solar radiation, and the back sheet being made of the same or a different material as the front sheet. Disposed between the front and back sheets so as to form a sandwich arrangement are the solar cells and a polymer material that encapsulates the solar cells and is also bonded to the front and back sheets. The laminated sandwich-style module is designed to mechanically support the brittle silicon cells and also to protect the cells against environmental degradation.
Photovoltaic solar cell modules having an effective working life of 30 years or more have been a well known industry objective. The materials used in constructing modules are selected with concern for providing adequate resistance to damage from impact and physical and thermal shock, maximizing the amount of solar radiation received by the cells, avoiding short-circuiting and electrical leakage, and otherwise minimizing degradation from such environmental factors as moisture, temperature and ultra-violet sunlight-induced chemical reactions. Furthermore, the thirty year useful life objective must be attainable at a commercially acceptable cost.
A particularly limiting factor in improving the useful life of solar cell modules has been the polymer materials used to encapsulate the cells. The tendency of the encapsulants to degrade under the influence of temperature and radiation has long been recognized as a critical problem. Thus prior to this invention, numerous materials were considered for use as encapsulants out of concern for increasing the useful life of solar modules. By way of example, a relatively large number of commercially available transparent polymer materials were surveyed and tabulated by B. Baum and M. Binette, in a 1983 report entitled "Solar Collectors" issued by Springborn Laboratories of Enfield, Conn. and describing an investigation conducted under Government Contract AC04-78CS35359. The polymers reviewed in that report covered a great variation in physical properties and chemistry. Among the polymer materials mentioned in the report were ethylene vinyl acetate copolymer (commonly known as "EVA") and ionomer. As a result of this investigation, twenty (20) transparent polymers were selected for possible use as encapsulants and/or glazings, and of these, EVA was recommended as the best encapsulant. Ionomer was not included in the twenty polymers selected for possible use.
In a second Springborn Laboratories report entitled "Investigation of Materials and Processes for Solar Cell Encapsulation", published August 1986 under JPL Contract 954527 S/L Project 6072.1 and reported by Paul B. Willis, four polymer materials were identified as being superior to all other materials investigated for use as encapsulants in solar cell modules. The four materials selected by the researchers were ethylene vinyl acetate coploymer (EVA), ethylene-methyl acrylate (EMA), butyl acrylate syrup, and aliphatic urethane chemistry polymers. However, EVA was identified in the report as the material offering the best combination of processability, performance, and low cost.
As a result of investigations such as that reflected by the second Springborn report, EVA has received wide commercial acceptance as an encapsulant for solar cell modules. Unfortunately, EVA has been found to be a less than ideal solar cell encapsulating material. For example, a well known problem associated with the use of EVA as an encapsulant is the occurrence of progressive darkening of the EVA under intense sunlight. This discoloration can result in a greater than 30% loss in power output of the solar module after only four or more years of exposure to the environment. This phenomena is well documented. See, for example, the article written by F. J. Pern and A. W. Czanderna of the Applied Sciences Branch, National Renewal Energy Laboratory, Golden, Colo., entitled, "Characterization Of Ethylene Vinyl Acetate (EVA) Encapsulation: Effects of thermal processing and weathering degradation on its discoloration", Solar Energy Materials and Solar Cells 25 (1992) 3-23 North-Holland, Elsevier Science Publishers.
The discoloration, which often is a yellow-brown color, results from the EVA chemically degrading under the influence of ultraviolet light. As the EVA decomposes, it releases acetic acid, which in turn acts like a catalyst in the EVA to cause further degradation. It has also been found that the degradation of the EVA may be further accelerated by heat and/or the presence of oxygen.
A further problem attendant to use of EVA as an encapsulant is that it requires use of a cross-linking agent, e.g., an organic peroxide, to achieve its desired properties as an encapsulant. Cross-linking deprives the EVA of the ability to be remelted. Thus, it is not possible to delaminate a module by heating so as to enable the in-process reworking of the module. Additionally, it is believed that any unreacted organic peroxide in the cross-linked EVA encapsulant will further assist the degradation process.
U.S. Pat. Nos. 4,239,555; 4,692,557; and 5,110,369 disclose a variety of solar cell encapsulation methods and encapsulated photovoltaic solar cell modules. Details of the construction of these encapsulated solar cell modules and their associated methods of manufacture are provided in the above-identified patents, which patents are hereby incorporated herein by reference. All of the foregoing patents disclose encapsulating materials that suffer from one or more limitations.